Two methods of imparting hydrophobic character to textiles have been investigated in the past: 1) hydrophobic polymer films, and 2) attachment of hydrophobic monomers and polymers via physi- or chemisorptive processes.
Current commercial processes for producing water-repellent/soil-resistant fabrics are mainly based on the laminating processes of companies such as W. L. Gore and Sympatex (Journal of Coated Fabrics vol. 26, 1996, pp. 107-130) and polysiloxane coatings (Handbook of Fiber Science and Technology, Marcel Dekker, New York, N.Y., Vol. II, 1984, pp. 168-171). The laminating process involves adhering a layer of polymeric material (such as Teflon™ that has been stretched to produce micropores) to a fabric. Although this process produces durable repellent films, it suffers from many disadvantages. The application of these laminants requires special equipment and therefore cannot be applied using existing textile production processes. Synthesis of the film is costly and garments with this modification are significantly more expensive than their unmodified counterparts. The colors and shades of this clothing are limited by the coating color. Finally, clothing made from this material tends to be heavy and stiff. Polysiloxane films suffer from low durability to laundering, which tends to swell the fabric and rupture the silicone film. The polysiloxanes have a cost advantage over the laminates, which are, however, more durable to laundering and dry-cleaning.
Repellents based on monomeric hydrocarbon hydrophobes can be broken down into five categories: 1) aluminum and zirconium soaps, 2) waxes and waxlike substances, 3) metal complexes, 4) pyridinium compounds, 5) methylol compounds, and 6) other fiber-reactive water repellents. Compared to polymeric coatings, monomeric hydrophobes can penetrate within the fabric to produce a more durable coating.
One of the oldest water repellents was based on non-covalently applying water-soluble soap to the fibers and precipitating it with an aluminum salt (J. Text. Res. vol. 42, 1951, p. 691). These coatings dissolved in alkaline detergent solution, therefore washfastness was poor. Zirconium soaps were less soluble in detergent solutions (Waterproofing and Water-Repellency, Elsevier Publ. Co., Amsterdam, 1963, p. 188), but due to the noncovalent nature of attachment to the fabric, abrasion resistance was poor.
The oldest and most economical way to make fabric water repellent is to coat it with a hydrophobic substance, such as paraffin (Text. Inst. Ind. vol. 4, 1966, p. 255). This process is still in practice today and paraffin emulsions for coating fabrics can be purchased (e.g., Freepel® from BFGoodrich Textile Chemicals, Inc.). Waxes are not stable to laundering or dry cleaning. Durability is poor due to their noncovalent nature of binding and their breathability is low.
Quilon chrome complexes polymerize to form —Cr—O—Cr— linkages (Tappi vol. 36, 1953, p. 107). Simultaneously, the complex forms covalent bonds with the surface of fibers with hydrophobic chains directed away from the surface to produce a water repellent, semi-durable coating. Quilon solutions require acidic conditions to react, thus causing degradation of the cellulose fibers through cellulose hydrolysis. Fabric colors are limited by the blue-green coloration imparted by the metal complex.
The extensive history of pyridinium-type water repellents has been reviewed by Harding (J. Text. Res. vol. 42, 1951, p. 691). In essence, an alkyl quaternary ammonium compound is reacted with cellulose at elevated temperatures to form a durable water-repellent finish on cotton (Br. Pat. 466,817) and a later version was marketed under the trademark Velan PF by ICI. It was later found that the reaction was restricted to the surface of the fibers (J. Soc. Dyers Colour. vol. 63, 1947, p. 260) and the high cure temperature weakened the fabric. Sodium acetate had to be added to prevent the decomposition of the cellulose by the HCl formed. Also, the pyridine liberated during the reaction has an unpleasant odor and the fabric had to be scoured after the cure. The toxicological properties of pyridine ended its use in the 1970s when government regulations on such substances increased.
Methylol chemistry has been extensively commercialized in the crosslinking of cellulose for durable press fabrics. N-methylol compounds are prepared by reaction of an amine or amide with formaldehyde. Alkyl-N-methylol compounds can be reacted at elevated temperatures in the presence of an acidic catalyst with the hydroxyl groups of textiles to impart durable hydrophobic qualities (Br. Pats. 463,300 and 679,811). The reaction is accompanied by formation of non-covalently linked (i.e., non-durable) resinous material, thus decreasing efficiency. In addition, the high temperature and acid catalyst reduce the strength of the fabric. Recently, the commercial use of methylol compounds has been waning due to concerns of toxic formaldehyde release from fabrics treated in such a manner.
Several other chemical reactions have been used to covalently attach hydrophobic species to cotton to produce a water-repellent finish but have not been commercialized for various reasons. Long-chain isocyanates have been used in this respect (Br. Pat. 461,179; Am. Dyest Rep. vol. 43,1954, p. 453; Br. Pat. 474,403). The high toxicity of isocyanates and significant side reactions with water, however, precluded it from commercial use. To circumvent the water sensitivity of isocyanates, alkyl isocyanates were reacted with ethylenimine to yield the less reactive aziridinyl compound, which was subsequently reacted with cellulose at 150° C. (Ger. Pat. 731,667; Br. Pat. 795,380). Although the toxicity of the aziridinyl compound was reduced compared to the isocyanate, the procedure still required the handling of toxic isocyanate precursors. Also, the high cure temperature weakened the cellulose, and crosslinkers were needed to increase structural stability. Alkyl epoxides can be reacted with cellulose under acidic or basic conditions to produce durable, water-repellent cotton (Ger. Pat. 874,289). The epoxide was applied from a volatile solvent to suppress side reactions with water. Epoxides are, in general, not very reactive, thus requiring long reaction times at high temperatures. Therefore, they have not been commercialized. Acylation of cotton with isopropenyl stearate from an acidic solution of benzene and curing at 200° C. produced a durable hydrophobic coating (U.S. Pat. No. 4,152,115). The high cure temperature and acid catalyst again weakened the cotton. Carcinogenic benzene can be replaced by toluene, but the practicality of using flammable solvents in fabric finishing is limited. Alkyl vinyl sulfones react with cellulose in the presence of alkali to form a repellent finish (U.S. Pat. No. 2,670,265). However, this method has not been commercialized because the alkali is not compatible with cross-linking reactants required for permanent press treatments.
Recently, copolymers containing a fluorinated monomer, an alkyl monomer, a reactive monomer (e.g., hydroxyethylmethacrylate, N-methylol acrylamide), and various other auxiliary monomers (e.g. vinylidene chloride, polyethylene glycol methacrylate, etc.) have become popular commercial products for the aqueous application of somewhat durable water and oil repellent finish to textiles (e.g., Zonyl™ by DuPont, Nuva™ by Clariant, and Scotchgard™ by 3M). These polymers, however, suffer from the release of formaldehyde from the treated fabric due to the breakdown of the N-methlyol acrylamide portion of the copolymer.
The use of mordants (insoluble metal complexes) have been used to permanently attach fluorinated compounds (containing groups such as acids capable of forming insoluble complexes with the mordant metal) to a textile substrate. The mordant approach of attaching the fluorinated compound to the substrate eliminates the use of the formaldehyde-releasing components described above. U.S. Pat. No. 3,651,105 uses a solvent-soluble fluorinated metal complex that is applied to paper and fabric. This complex is monomeric, so it only has one point of attachment as opposed to multiple attachment points afforded by a polymer. But, more importantly, this complex is only soluble in a carbon tetrachloride/isopropanol mixture. The use of toxic and flammable solvents in a textile process is impractical. Water-soluble complexes are preferred. U.S. Pat. No. 3,467,612 uses a water-soluble fluoropolymer/metal complex but the fluoropolymer does not contain any monomers capable of complexing with a divalent metal to bridge to a substrate. However, although the fluorinated complex is insolubilized on the fabric, it is not directly bound to a group on the substrate; thus durability to abrasion is low. Other patents using divalent metals use a two-step process where the metal is applied to the fabric first and then the fluoropolymer containing a monomer capable of complexing a divalent metal is applied to the fabric in a second step. EP 0710738 teaches a two-step process: application of a divalent metal followed by application of a random fluoro-copolymer (containing monomers capable of binding the divalent metal). The use of two steps greatly decreases the utility of this approach due to cost issues. U.S. Pat. No. 5,744201 uses a water-soluble random fluoro-copolymer with an acid-containing monomer that is rendered insoluble (and thus precipitated on the fabric) by changing the pH in the presence of ammonium ion (single valent). The copolymer and the ammonium ion form an insoluble complex at a specific pH and is not directly bound to a group on the substrate; thus durability to abrasion is low. EP 572269A1 is similar except for the use a polyallylamine salt instead of ammonium. This patent also mentions the use of monomeric fluorinated zirconium compounds as additives to boost performance. U.S. Pat. No. 4,695,488 incorporates acrylic acid in their fluoropolymer to increase soil release properties but do not use this acid group to form insoluble divalent salt linkages to the fabric substrate. The purpose of the acid groups is to increase the soil-release properties of the finish.